In many wireless communications systems, the power amplifier (PA) in the transmitter is required to be very linear, in addition to being able to simultaneously amplify many radio channels (frequencies) spread across a fairly wide bandwidth. It also has to do this efficiently, in order to reduce power consumption and need for cooling, and to increase its longevity. High linearity is required since nonlinear amplifiers would cause leakage of interfering signal energy between channels.
The amplitude probability density of a mix of sufficiently many independent radio frequency (RF) channels, or of a multi-user CDMA (Code Division Multiple Access) signal, tends to be close to a Rayleigh distribution having a large peak-to-average power ratio. Since a conventional RF power amplifier generally has an efficiency proportional to its output amplitude, its average efficiency is very low for such signals.
In response to the low efficiency of conventional linear power amplifiers, many methods have been proposed. Two of the most promising are the Chireix outphasing method [1] and the Doherty method [2]. Since their publication in 1935 and 1936, respectively, the theories, implementations and improvements of these two types of RF amplifier systems have remained separate from each other. The operation of the Chireix amplifier has thus been described in terms of constant-voltage operation of the constituent amplifiers, with outphasing of the constant-amplitude phasors as the method for obtaining amplitude modulation. The efficiency increasing action of the Doherty amplifier has been described in terms of (dynamic) impedance modulation of the load or, equivalently, variable load distribution.
At least three problems have been identified in these previously known composite power amplifiers.
Firstly, the output network of Chireix and Doherty amplifiers must be tuned with high precision to have the right properties at the desired frequency. This is a costly and time-consuming process, especially when large quantities of transmitters are to be produced.
Secondly, since their respective implementations are very different, the choice between making a Doherty and a Chireix amplifier must be made very early in the design process. It is also hard to change this decision later, since this involves a lot of redesign.
Thirdly, neither the Chireix nor the Doherty amplifier is suited for operation over multiple, relatively closely spaced bands (such as 1800/1900/2100 MHz).
In light of the problems just described, there is a need for an amplifier that can operate without excessive tuning of the output network. There is also a need for an amplifier platform, wherein the choice between a Chireix and a Doherty implementation can be made late in the design process, to match the advantages of the respective amplifying system with the requirements of the application. A need can also be identified for an amplifier that can function over multiple bands without redesign or retuning.